Generally, in a synchronous generator, a direct current is supplied to its rotor's field winding, and induced electromotive force generated in accordance with magnetic flux produced on the rotor side is outputted on a stator side. As representative methods for energizing the rotor's field winding, there are a thyristor excitation scheme and an AC brushless excitation scheme.
Between the schemes, the brushless excitation scheme is a method in which a rotary rectifier rectifies electric currents induced in armature windings of an AC exciter constituted of a field winding(s) on its stator side and, on its rotor side, the armature windings directly connected to a rotational shaft of a generator, and a rectified current is supplied to a field winding of the generator. In general, at the time of a generator's nominal operation, a voltage applied to the field winding(s) of the AC exciter is that of direct current.
As a start-up operation method of a synchronous generator, there is a method in which, using a static frequency converter (SFC: Static Frequency Converter; hereinafter, referred to as an “SFC,” for brevity), the generator is accelerated by gradually raising a frequency of armature currents of the generator, namely, which is accelerated as an electric motor.
In the brushless excitation scheme, when a voltage applied to field windings of an AC exciter is a direct current and the field current thereof is maintained constant, a field current of a generator cannot be flowed during the halts of the generator, and the field current of the generator changes responding to a generator speed. For this reason, at the time of starting the generator, DC excitation is not applicable to an AC exciter.
When a start-up operation takes place using an SFC, there is a method in which, in order to use an AC exciter, a wound-rotor induction motor is used as the AC exciter (for example, refer to Patent Document 1).
When the number of revolutions of a generator is lower than or equal to a predetermined value, AC voltages are applied to field windings of the AC exciter; and, when the number is larger than a predetermined value, connections to the field windings of the AC exciter are modified, and, to the field windings thereof, a DC voltage is applied from a thyristor excitation device. In addition, there is a method in which, at the time of starting a generator, AC magnetic flux configured in a reverse direction to a stator side thereof is produced in three-phase windings of an AC exciter (for example, refer to Patent Document 2). Moreover, there is a method in which an SFC for use in a brushless exciter is provided to be used at low rotational speeds (for example, refer to Patent Document 3). The SFC for use in the brushless exciter applies electric currents of constant amplitudes to stator field windings of the exciter at such a frequency that a relative rotational speed of a rotating magnetic field viewed from the rotor is always maintained at a nominal rotational speed.